A multi-frequency video display monitor/television receiver having a cathode ray tube (CRT) is capable of operating at, for example, a horizontal deflection frequency selected from a range of frequencies in accordance with, for example, a selection controlled by a user, referred to as mode change. It may be advantageous to keep the ultor voltage of the CRT present, during the mode change, so that the picture will return as soon as possible after the mode change. It may also be desirable to disable, for example, the horizontal deflection current for protecting a horizontal output transistor, during the mode change. In order to reduce the possibility of phosphor burns of the CRT screen in the absence of horizontal deflection, it may be desirable to blank the CRT screen, during the mode change.
In the event of improper blanking adjustment, for example, during a field service, the CRT screen might not be completely blanked, during the mode change. Therefore, a CRT screen burn could occur that causes damage to the CRT. It may be desirable to reduce the possibility of improper adjustment.
In carrying out an inventive feature, an ultor voltage switching power supply is inhibited from switching, from a time that occurs, for example, approximately 120 millisecond prior to an interval when the deflection current is disabled and during the interval when the deflection current is disabled. An ultor electrode capacitance of the CRT is partially discharged via a large bleeder resistor, when the ultor voltage switching power supply is inhibited from switching. As a result, the ultor voltage exhibits a transient decrease to a fraction of its normal operation value. The transient decrease, advantageously, maintains the CRT screen blanked even when improper screen blanking control occurs.